1. Field of the Invention
This invention relates to a sheet metal pump casing and its fabrication method and particularly to a pump casing made by roll forming of sheet metal, and having a flange reinforced structure to enhance pump casing strength, and having a guide plate for forming more effective volute.
2. Description of the Prior Art
Conventional methods for fabricating a pump casing for a centrifugal pump generally include casting and sheet metal working. The pump casing made by casting usually is relatively bulky and heavy. It takes more material and space. Because of material characteristics (cast iron), it has poor erosion resistantance and low stiffness. It is therefore less durable and is mostly used in relatively low value and price products. The pump casing made by sheet metal stamping may use high strength and high erosion resistant metal (such as stainless or alloy steel). It is lighter and smaller and more durable. However due to pump casing made by sheet metal is relatively thin, it is prone to deform under external force. Furthermore during stamping process, high stress concentration often happens at some spots of the pump casing and results in strength reduction, uneven thickness, deformation or even fracture.
In general, a pump connects with a piping system at the inlet and outlet. The piping system constantly subjects to heat expansion and cold contraction effects. The fluid pressure has wide variation. The pump impeller will generate torque during rotation. All these factors will create dynamic loads acting upon the inlet and outlet, and could result in deformation of pump casing at the inlet and outlet sections. It is therefore a constant studying subject in pump design on how to increase the strength of the inlet and outlet sections to withstand those internal and external forces and stress. PCT (Germany) patent No. PCT/DE86/00188 discloses a sheet metal pump design (shown in FIG. 1). The centrifugal pump 10 has a sheet metal pump casing 11 which is formed by a sheet metal working piece or by welding a plural number of sheet metal working pieces together. The sheet metal working pieces are made by stamping processes. The pump casing 11 near the inlet 12 is formed in ravine shape for reinforcement. However there is still some deficiencies in such construction. For instance, it is generally known that the centrifugal pump usually has a wear ring 15 located at the front end of the impeller 14 near the inlet 12. It is to keep clearance between the impeller tip and the inside wall of the pump casing 11 so that when the impeller rotates in high speed, the severe friction that might otherwise happen between the impeller and the pump casing may be avoided. The degree of clearance directly affects pump efficiency, noise level and working life of the pump. Although the pump 10 shown in FIG. 1 has reinforcement structure for the pump casing 11, the maximum force receiving location is still at the inlet 12 section. When the pump 10 subjects to high stress, torsion or torque, the pump casing 11 near the inlet 12 is easily deformed. The wear ring 15 clearance is prone to change. It could result in lower pumping efficiency or increasing noise level, or even damage the pump. It is also known that a volute will be formed between the impeller 14 and the inside wall of the pump casing 11. The volute will produce better pumping effect with increasing crosssection dimensions starting from the outlet along the rotor 14 rotation direction. This variating dimension shape is easily made by casting method but is difficult to form by stamping sheet metal working process. Therefore, in FIG. 1, the volute has a constant dimension rather than variation that is more desirable. The pumping efficiency will be lowered because of such deficiency.
European patent No. EPO-0442070A1 (Ghiotto) discloses another centrifugal pump 20 with sheet metal pump casing 21 (shown in FIG. 2). Around the inlet 22 is a flange 23. Near the flange 23 at the inlet section, there is a reinforced member 26 formed in a passage shape and relatively greater thickness than the pump casing. The reinforced member 26 is welded to the inside wall of the pump casing 21 to support high stress at the inlet section. However when the flange 23 is subjected to great external force, the wear ring clearance at the front end of the impeller 25 tends to change. Hence it will cause dropping of pumping efficiency, increasing noise level and reducing working life. Furthermore it also does not cover detailed description about the volute 27 improvement.
There have been prior cases which tried to employ multiple stamping processes on sheet metal along the volute of the pump casing for producing increasing crosssections volute. They usually need a plural number of dies and stamping operations. The total fabrication time is long and the cost is higher. Moreover excessive stamping processes could result in harmful effect on material property and structure of the pumping casing.
In view of aforesaid disadvantages, it is therefore an object of this invention to provide a sheet metal pump casing and its fabrication method for producing sheet metal pump casing with greater strength. It can also result in stable wear ring clearance at the front of the impeller and volute with increasing crosssections at low cost. The sheet metal working method according to this invention can form a pump casing with stable and steady strength without undue stress concentration or downgrade the pump casing material.
The sheet metal pump casing according to this invention includes a body, an impeller, an inlet flange and an outlet flange. The body has an inlet engaging with the inlet flange, and an outlet engaging with the outlet flange. At the inlet flange, there is provided with a first flap flange which has one end located from the peripheral of the inlet flange and another end extending to the outside wall of the body and spaced from the inlet. Around the outlet flange, there is also a second flap flange with one end extending to the outside wall of the body and connecting with the inlet flange. Because of such construction, the stress, torque and torsion applying on the inlet flange and outlet flange will not directly transmit to the body near the inlet and outlet. The first and second flap flange will receive and disperse the stress and forces to other parts of the body remote from the inlet and outlet. Therefore the wear ring clearance at the front end of the impeller will be least affected by external force.
According to one aspect of this invention, the first and second flap flange are respectively made in a double flange form and welded to body in opposite direction between the inlet (or outlet) flange and the body. The flap flange has a flap foot fixed to the body. The flap foot has the contour mating with the shape and form of the body so that the stress and force may be dispersed more evenly.
In another aspect of this invention, there is provided with a guide plate formed in an arched shape and being fixed to the inside wall of the body. The guide plate has a joint section mating and welded to the inside wall near the outlet, a cut water section and a spread section. The cut water section bridges between the joint section and the spread section with a cut water end projecting toward the impeller but spaced from it. The spread section is an elongated narrow strip with a head end connecting with the cut water section and a tail end fixing to the inside wall of the body. The middle section of the spread section is formed in curved shape around the impeller with a narrow gap at the cut water end and wider gap at the tail end. Therefore the impeller and the guide plate form a volute with increasing crosssection from the cut water end to the tail end. When the impeller rotates and pumps fluid, fluid flows in the volute and being discharged out from the outlet more smoothly. The cut water section also helps to smooth out fluid discharge through the outlet.
In yet another aspect of this invention, this invention provides a fabrication method for forming a sheet metal pump casing with above mentioned structure. The body of the pump casing is made of a basin-shaped metal. By using at least one set of roller dies in geometric shape to roll forming the basin-shaped metal, the body of the pump casing is formed with an inlet end plate, a passage side plate and a neck section. Because the roller dies form sheet metal gradually and smoothly, the stress concentration can be minimized. The harmful effect of stress concentration that occurs in conventional stamping process thus may be avoided.
In a further aspect of this invention, the aforesaid inlet end plate is made separately by a thick or high strength sheet metal and being welded to the body at the inlet section. The body, particularly at the inlet section thus may withstand higher stress and force.